Every element in an array consists of a key => value pair. Depending on how the array has been created this will either be a numeric key based on the position of the element in the array (starting at zero) or a name.

In each case the $key is the numeric or named index and the $value is the value of each item.

To go through elements in an array you use for or foreach, so in your example it is saying for each element in the array $beatles extract each element's key and value (this is where the 'as' comes into play) and echo out the data.